Atmosphere-Pressure Methane Oxidation to Methyl Trifluoroacetate Enabled by a Porous Organic Polymer-Supported Single-Site Palladium Catalyst

Article abstract of DOI:10.1021/acscatal.0c05205

The efficient conversion of methane into methanol at low temperature under low pressure remains a great challenge largely because of the inertness and poor solubility of methane. Herein, we report that a porous organic polymer-supported Pd catalyst, which was constructed via Friedel-Crafts type polymerization between 4,6-dichloropyrimidine and 1,3,5-triphenyl benzene and subsequent metalation, enabled the conversion of methane to methyl trifluoroacetate, a precursor to methanol, under atmosphere pressure (1 atm) at 80 °C to afford a 51% yield relative to methane with a TON of 664 over 20 h. On increasing the pressure to 30 bar, this palladium catalyst offered a TON of 1276 for a run and could be reused for at least five runs without a notable loss of activity. The characterization of this Pd catalyst revealed its good affinity for methane uptake that would increase the concentration of methane in the local space around the Pd center and the homogeneous distribution of Pd2+ on support that would protect the catalytically active metal species, shedding light on the high catalytic activity of this Pd catalyst toward methane conversion.

Full text of DOI:10.1021/acscatal.0c05205

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Letter
Atmosphere-Pressure Methane Oxidation to Methyl Trifluoroacetate
Enabled by a Porous Organic Polymer-Supported Single-Site
Palladium Catalyst
Yiwen Zhang,⊥ Min Zhang,⊥ Zhengbo Han, Shijun Huang, Daqiang Yuan,* and Weiping Su*
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ABSTRACT: The efficient conversion of methane into methanol
at low temperature under low pressure remains a great challenge
largely because of the inertness and poor solubility of methane.
Herein, we report that a porous organic polymer-supported Pd
catalyst, which was constructed via Friedel−Crafts type polymer-
ization between 4,6-dichloropyrimidine and 1,3,5-triphenyl ben-
zene and subsequent metalation, enabled the conversion of
methane to methyl trifluoroacetate, a precursor to methanol,
under atmosphere pressure (1 atm) at 80 °C to afford a 51% yield
relative to methane with a TON of 664 over 20 h. On increasing
the pressure to 30 bar, this palladium catalyst offered a TON of
1276 for a run and could be reused for at least five runs without a
notable loss of activity. The characterization of this Pd catalyst
revealed its good affinity for methane uptake that would increase the concentration of methane in the local space around the Pd
center and the homogeneous distribution of Pd2+ on support that would protect the catalytically active metal species, shedding light
on the high catalytic activity of this Pd catalyst toward methane conversion.
KEYWORDS: porous organic polymer, methane, oxidation, methyl trifluoroacetate, palladium catalyst
ethane, the main component of natural gas, is an
devoted great efforts to the development of the catalytic
methods for direct conversion of methane to methanol or
methanol derivatives via selective oxidation.14,23−38 These
elegant studies have disclosed a variety of factors that are
responsible for the catalytic efficiency, including ligand effects
on the activity, selectivity and lifetime of catalysts,27,29,32 the
effects of nature and concentration of oxidants on catalysis,28,30
the dependence of the reaction types on the intrinsic
properties of metal catalysts,26,31,35 and the developed strategy
to avoid overoxidation of methanol product.25 These findings
have led to the improvement of the activity and lifetime of
catalysts and reaction selectivity, representing significant
advances in this area. However, there is still room for the
development of a catalyst system to achieve the efficient
conversion of methane to methanol under mild conditions,
since the existing methods generally require high temperature
(up to 200 °C) and high pressure (typically >30 bar). The
1−3
Mabundant, inexpensive resource of hydrocarbon.
So
far, practical conversions of methane into synthetic fuels and
fine-chemicals have all relied on the high-temperature, high-
pressure multistep processes that consist of the initial steam
reforming to generate syngas (a mixture of carbon monoxide
and hydrogen) and subsequent transformations of syngas to
targeted products.4−6 Over the past decades, the direct
conversion of methane to value-added products without the
reliance of formation of intermediate syngas under mild
conditions (e.g., low-temperature and low-pressure) has been
the sought-after goal of chemists from both academic and
industrial communities,7−14 because the achievement of such a
goal would provide low-energy, cost-effective processes, and
allow realizing the potential utility of widely available methane
as an industrial feedstock. As a result, great advances have been
achieved in the development of the strategies for addressing
the challenges posed by the intrinsic inertness of methane that
stems from the strong C−H bond, nonpolarity of the molecule,
poor solubility in most solvents, and low acidity.15−35
Nevertheless, the efficient catalysts for the direct conversion
of methane under mild conditions remain in high
demand.12−14
Received: November 27, 2020
Revised: December 25, 2020
Published: January 7, 2021
Realizing that methanol is a versatile feedstock for
production of various fine chemicals,36−38 chemists have
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Letter
homogeneous trifluoroacetic acid based systems, which employ
highly stable bis(NHC) palladium,27 manganese oxide,39
cobalt acetate,40 or copper oxide31 as the catalyst, allow the
selective oxidation of methane to methyl trifluoroacetate to
occur under realtively mild conditions but with a low turnover
number (TON).
methane under such low pressure has been reported before our
work. When CH4 pressure was increased to 30 atm, this
reaction obtained a TON of 1276 over 20 h. This POPs-
supported palladium catalyst did not show a notable decrease
in catalytic activity after being reused at least 5 times in the
conversion of methane to methyl trifluoroacetate. Our findings
demonstrate the great potential of MOFs/POPs-supported
catalysts to affect the challenging conversions of inert gaseous
substrates under mild conditions.
Our investigations began with preparation and character-
ization of POPs-supported metal catalysts. Recently, a
microporous covalent triazine-based organic polymer that is
constructed from cross-coupling between 1,3,5-trichloro
triazine and 1,3,5-triphenyl benzene, proved to be an adsorbent
for CH4 with pretty good capture capacity.63 In light of this, we
synthesized an analogous POPs material using the method
previously established by us64 to check its utility as catalyst
support. As shown in Figure 2, the designed POPs material was
Recently, metal−organic frameworks (MOFs) as well as
porous organic polymers (POPs) have been widely used as
versatile and tunable catalyst supports to construct single-site
metal catalysts.21,22,32−35,41−59 The pioneering studies in this
area have demonstrated that the separation of metal catalyst
within porous frameworks protects highly catalytically active,
coordinatively unsaturated metal species from agglomeration
and decomposition and therefore endows active catalysts with
the high stability that is inaccessible in homogeneous catalysis
systems.22,41,45,46 Importantly, depending on pore size and
hydrophilic/hydrophobic nature, MOFs and POPs are capable
of selectively adsorbing organic compounds including gaseous
substances.60−62 These achievements inspired us to consider
whether MOFs/POPs-supported solid catalysts could realize
the direct conversion of methane to methanol under mild
conditions. The desired process combines the high activity of
homogeneous catalysts and the ease of heterogeneous catalysts
and thus would exhibit an excellent overall performance. The
potential advantages of the MOFs/POPs-supported solid
catalysts for methane conversion, we speculated, lie in the
following two aspects. First, the hydrophobic open-pores of
MOFs or POPs may adsorb methane gas60,62 and thus
significantly increase the concentration of methane in the local
space around the catalyst centers immobilized within pores,
which would lead to a decrease in the activation entropy of
methane and allow for the reaction under relatively low
pressure. Second, the choice of the proper ligand moiety
tethered to MOFs or POPs frameworks may give rise to the
catalytically active metal catalysts41 that, in combination with
the decreased activation entropy, enable the reaction to occur
at relatively low temperatures and extend the lifetimes of
catalysts because of the isolation of metal centers of catalysts.22
Herein, we report a POPs-supported palladium catalyst that
enables the oxidative conversion of methane to methyl
trifluoroacetate at 80 °C under pressure of 1 atm with the
turnover number (TON) of 664 reached over 20 h (Figure 1).
To the best of our knowledge, no method for the conversion of
Figure 2. Construction of porous organic polymer incorporating
pyrimidine units (Pyr-POPs) and palladation of Pyr-POPs to Pyr-
POPs-Pd.
prepared via AlCl3-mediated Friedel−Crafts arylation of 4,6-
dichloropyrimidine with 1,3,5-triphenyl benzene that was
conducted at 40 °C in methylene chloride for 20 h and
isolated as a solid powder (denoted as Pyr-POPs) after
purification (for experimental details, see Supporting Informa-
tion). Then, Pyr-POPs was postsynthetically metalated via
nitrogen ligand-assisted cyclopalladation using Na2PdCl4 as a
palladium source to generate Pd-incorporating porous organic
polymer, Pyr-POPs-Pd.
The successful synthesis of Pyr-POPs was supported by
several structure determination approaches. The solid-state 13
C
NMR spectrum of Pyr-POPs shows the peaks at 169 and 162
ppm that can be assigned to carbon atoms at the pyrimidine
ring and the broad peaks at around 139 and 128 ppm
corresponding to carbon atoms of phenyl rings, suggesting that
both pyrimidine and 1,3,5-triphenyl benzene moieties are
incorporated into the Pyr-POPs framework (Figure S1).
Nevertheless, a peak at 35 ppm in this 13C NMR spectrum
reflects the incorporation of methylene carbon into the Pyr-
POPs due in large part to the AlCl3-mediated Friedel−Crafts
alkylation reaction of 1,3,5-triphenyl benzene with methylene
chloride solvent, which is consistent with the elemental
analysis of Pyr-POPs sample that is slightly different from
the values calculated on the basis of the expected structure of
Pyr-POPs (Figure 2). The Fourier-transform infrared (FTIR)
spectrum (Figure S2, S3) provides the structural information
similar to its 13C NMR spectrum. The bands at 1506 and 1375
cm−1 appearing in the spectrum of Pyr-POPs may result from
the CN and C−N stretching vibrations of the pyrimidine
ring. Besides, the disappearing C−Cl stretching band at 850
cm−1 in the FTIR spectrum confirms the complete conversion
of pyrimidinyl chloride.65
Figure 1. Methane oxidation to methyl trifluoroacetate. (a)
Conventional conversion of CH4 with homogeneous catalysts. (b)
Conversion of CH4 with porous organic catalyst−supported catalyst
(this work).
X-ray photoelectron spectroscopy analysis validated that the
elemental compositions of Pyr-POPs-Pd are C, N, Cl, and Pd
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elements (Figure S4). Moreover, the XPS spectrum of Pyr-
POPs-Pd revealed that the binding energy peaks at 337.6 and
342.8 eV are characteristic signals of the Pd 3d5/2 and Pd 3d3/2
corresponding to PdII oxidation state (Figure S5). In
comparison with the Na2PdCl4 (338.1 and 343.3 eV), the
PdII binding energy in the Pyr-POPs-Pd shifted negatively by
0.5 eV, which can be attributed to the strong electron-donation
of the Pyr-POPs framework. These results confirm that the
PdII is successfully immobilized on the Pyr-POPs by
coordination to pyridine functional groups rather than by
physical adsorption of Na2PdCl4 on the surface.66
The combined TEM/EDX analysis (TEM, transmission
electron microscopy; EDX, energy-dispersive X-ray spectros-
copy) indicated a very homogeneous palladium loading
without detectable palladium clusters or nanoparticles within
Pyr-POPs-Pd framework. The high-annular dark-field scan-
ning TEM and EDX mapping spectroscopy also supported the
uniform inclusion of the carbon, nitrogen, palladium, and
chlorine center into the polymer frameworks (Figure S6).
Bright spots marked with red circles in the aberration-
corrected HAADF-STEM image (Figure S8) arose from
isolated atoms, which indicated that the agglomeration of
Pd(II) ions might not occur during the impregnation step. A
broad peak centered around 20° in the powder X-ray
diffraction pattern (Figure S9) indicates the amorphous
character of the Pyr-POPs and Pyr-POPs-Pd material that
consists of particles with sizes ranging from 10 to 30 μm as
shown by the field-emission scanning electron microscopy
image (Figure S10).
is shown to have an N2 uptake of 325 cm3g−1 with a
Brunauer−Emmett−Teller (BET) surface area of 812 m2g−1
(pore volume 0.50 cm3 g−1), while Pyr-POPs-Pd demonstrates
a smaller N2 uptake of 248 cm3g−1 with a BET surface area of
671 m2g−1 (pore volume 0.37 cm3 g−1). The pore-size
distributions were calculated by the density functional theory
model (Figure 3a, inset). On the basis of these calculations,
Pyr-POPs-Pd was revealed to have two types of micropores
with pore widths of 0.54 and 0.64 nm, which are smaller than
those of Pyr-POPs with pore widths of 0.68 and 0.79 nm as a
result of incorporation of Pd species into Pyr-POPs porous
framework. Importantly, Pyr-POPs and Pyr-POPs-Pd exhibit
a good affinity for CH4 with CH4 uptake of 29.4 and 23.4
cm3g−1 at 273 K, 25.0 and 19.4 cm3g−1 at 283 K, and 20.6 and
15.9 cm3g−1 at 293 K, respectively (Figure 3b,c). To probe the
host-CH4 binding affinity, the isosteric heats of adsorption
(Qst) are calculated on the basis of the CH4-adsorption
isotherms at 273, 283, and 293 K (Figure 3d). The high Qst
values derived from these data point to the good affinity
between CH4 and the hydrophobic walls of micropores.
With Pd-incorporating porous organic polymer Pyr-POPs-
Pd in hand, we evaluated the catalytic activity of Pyr-POPs-Pd
toward the direct conversion of methane under mild
conditions using the selective oxidation of methane to generate
methyl trifluoroacetate as a model system (Table 1). The
Table 1. Effects of Reaction Parameters on the Catalytic
a
Efficiency of the CH4 Oxidation to Methyl Trifluoroacetate
The nitrogen sorption measurements demonstrate micro-
porous structures of Pyr-POPs-Pd and Pyr-POPs samples. As
shown in Figure 3a, both the samples display a classical type I
isotherm with a sharp uptake at low relative pressure (P/P°<
0.1), reflecting abundant micropores in frameworks of Pyr-
POPs-Pd and Pyr-POPs. Actually, the micropores of suitable
size in heterogeneous catalysts are beneficial to mass transport
for reactants and products during catalysis process. Pyr-POPs
methane
(bar)
CF3CO2Me
(mM)
b
entry
catalyst
TON
1
2
3
4
5
Pyr-POPs-Pd
Pyr-POPs-Pd
30
1
30
30
30
102.14
53.14
13.33
26.25
45.36
1276
c
664
d
[Pd(2-PhPy)(μ-Cl)]2
167
328
567
e
Na2PdCl4
e
Na2PdCl4 +Pyr-POPs
a
Reaction conditions: 1 mg of catalyst with 5.95 wt % Pd content, 570
mg of K2S2O8(2.1 mmol), 6 mL of TFA, 1 mL of (TFA)2O, 20 h, 80
b
°C, under the indicated CH4 pressure (25 °C). TON was calculated
c
as the molar ratio of CF3CO2Me to the palladium content. 51% yield
based on methane was obtained as methane (0.73 mmol) is the
d
e
limiting reagent under atmosphere pressure. 0.28 μmol catalyst. 10
μL stock solution of NaPdCl4 in H2O (0.056 M).
model reaction was carried out in a mixed solvent of
trifluoroacetic acid and trifluoroacetic acid anhydride at 80
°C for 20 h under the indicated CH4 pressure using K2S2O8 as
an oxidant and an autoclave as a reactor. To our delight, 1 mg
of Pyr-POPs-Pd with 5.95 wt % palladium content was
observed to be a high active catalyst for the conversion of
methane to methyl trifluoroacetate under 30 bar of CH4
pressure, affording a TON as high as 1276 (entry 1).
Interestingly, this porous framework-supported Pd catalyst
enabled the reaction to take place under atmosphere pressure
(1 atm) in a 51% yield relative to CH4 with a TON of 664
(entry 2). To evaluate the effect of porous framework support
on the catalytic activity of the catalyst, a molecular bimetallic
palladium complex, which is similar in the coordination sphere
of palladium center to Pyr-POPs-Pd, was tested as a catalyst
for the model reaction under 30 bar of CH4 pressure (entry 3).
This chloride-bridged bimetallic palladium complex bearing a
Figure 3. (a) Nitrogen sorption isotherms of Pyr-POPs and Pyr-
POPs-Pd measured at 77 K. Inset: Pore size distributions of Pyr-
POPs (blue) and Pyr-POPs-Pd (red). (b, c) CH4 adsorption
isotherms of Pyr-POPs (b) and Pyr-POPs-Pd (c) collected at 273,
283, and 293 K. (d) Heats of CH4 adsorption for Pyr-POPs and Pyr-
POPs-Pd at 273, 283, and 293 K.
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cyclopalladated 2-phenylpyridine ligand was observed to
promote the reaction but offer a much lower TON (167)
than Pyr-POPs-Pd, illustrating the positive effect of porous
framework support Pyr-POPs. NaPdCl4 also effected the
conversion of methane with a TON of 328 (entry 4), while
introduction of Pyr-POPs into the reaction system with
NaPdCl4 to in situ generate the Pyr-POPs-Pd led to the
improvement of TON (576) (entry 5), once again indicating
the positive role of Pyr-POPs in the enhancement of catalyst
activity. The significantly decreased TON value with in situ
generation of Pyr-POPs-Pd compared with the case of
presynthesized Pyr-POPs-Pd is likely due to incomplete
incorporation of Pd salts into POPs framework.
The insight into the mechanism for this direct conversion of
methane was obtained by the free-radical trapping experiment
in which introducing radical scavengers such as benzophenone,
hydroquinone, and phenol into the reaction system shut down
the reaction of methane. This radical trapping experiment
suggested the reaction involved formation of free-radical
intermediate. Next, the recyclability of this heterogeneous
catalyst Pyr-POPs-Pd was investigated (Figure 4). The used
by the Pyr-POPs-Pd enabled selective oxidation of methane
by K2S2O8 under atmosphere pressure (1 atm) that offered
methyl trifluoroacetate in a 51% yield relative to CH4 with a
TON of 664. When pressure was increased to 30 bar, Pyr-
POPs-Pd provided a TON as high as 1276 and could be
reused for at least 5 runs without significant loss of activity,
indicative of its good stability. The observation that Pyr-POPs-
Pd outperformed the molecular palladium complex with
similar coordination sphere in terms of both catalytic efficiency
and reaction conditions implicates that the ability of porous
framework to adsorb methane and the isolation of Pd centers
within Pyr-POPs-Pd may both play key roles in enhancement
of its catalytic activity. On the basis of the tunability of
porosity, chirality, hydrophilic/hydrophobic nature and chem-
ical functionality of porous frameworks, the design and
development of porous framework-supported metal catalysts
for efficient chemical conversions, particularly with inert but
readily available starting materials is underway in our
laboratories.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05205.
Synthesis and characterizations for Pyr-POPs and Pyr-
POPs-Pd, detailed procedures, and characterizations
data of catalytic methane oxidation (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
Daqiang Yuan − State Key Laboratory of Structural
Chemistry, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou 350002,
China; University of the Chinese Academy of Sciences, Beijing
100049, China; orcid.org/0000-0003-4627-072X;
Email: ydq@fjirsm.ac.cn
Figure 4. Evaluation of the recyclability of Pyr-POPs-Pd catalyst in
the CH4 oxidation to methyl trifluoroacetate. All reactions were
conducted under the conditions similar to those described in Table 1,
entry1.
Weiping Su − State Key Laboratory of Structural Chemistry,
Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou 350002, China;
University of the Chinese Academy of Sciences, Beijing
100049, China; orcid.org/0000-0001-5695-6333;
Email: wpsu@fjirsm.ac.cn
Pyr-POPs-Pd catalyst, which was isolated from the first-run
reaction system by centrifugation and filtration and sub-
sequently purified by washing with water and ketone, was
found to be still active toward the conversion of methane to
methyl trifluoroacetate under the standard reaction conditions.
This Pyr-POPs-Pd catalyst proved to be reusable for at least
five iterative runs without notable loss of activity, highlighting
its stability against decomposition under acidic conditions. We
carried out TEM images, HAADF-STEM image, EDS
elemental mapping spectroscopy (Figure S11) as well as
aberration-corrected HAADF-STEM image (Figure S13) of
recycled catalyst, and it can be seen that the metal palladium in
recovered catalyst is stable. Moreover, the solid-phase 13C
NMR spectra of recyclable catalyst were mostly maintained
(Figure S14). These observations suggest that the stability and
structural integrity of the Pyr-POPs-Pd catalyst could with-
stand the test, thus providing potential to practical application.
In summary, we have synthesized a single-site palladium
catalyst supported on porous organic polymers framework that
features microporous structure, high BET surface area, and
good affinity for methane uptake. This porous organic
polymer-supported single-site palladium heterogeneous cata-
lysts Pyr-POPs-Pd proved to be active toward the direct
conversion of methane under mild conditions, as exemplified
Authors
Yiwen Zhang − State Key Laboratory of Structural Chemistry,
Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou 350002, China;
College of Chemistry, Liaoning University, Shenyang 110036,
China
Min Zhang − State Key Laboratory of Structural Chemistry,
Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou 350002, China;
orcid.org/0000-0002-4344-9815
Zhengbo Han − College of Chemistry, Liaoning University,
Shenyang 110036, China; orcid.org/0000-0001-8635-
9783
Shijun Huang − State Key Laboratory of Structural
Chemistry, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou 350002, China
Complete contact information is available at:
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Author Contributions
⊥(Y.Z., M.Z.) These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Key Research and
Development Program of China (2018YFA0704502), the
National Natural Science Foundation of China (Grants No.
21931011, 22071241, and u1505242), the Strategic Priority
Research Program of the Chinese Academy of Sciences
(XDB20000000 and XDB10040304), and the Key Research
Program of Frontier Sciences, CAS (QYZDJ-SSW-SLH024).
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